A comparative study of low cost biomaterials for the removal of chromium (VI III) from aqueous solutions

A COMPARITIVE STUDY OF LOW-COST BIOMATERIALS
FOR THE REMOVAL OF CHROMIUM (VI/III) FROM
AQUEOUS SOLUTIONS





SYAM KUMAR PRABHAKARAN










NATIONAL UNIVERSITY OF SINGAPORE
2006

A COMPARITIVE STUDY OF LOW-COST BIOMATERIALS
FOR THE REMOVAL OF CHROMIUM (VI/III) FROM
AQUEOUS SOLUTIONS




SYAM KUMAR PRABHAKARAN

(B.Tech. (Hons.), NUS)




A THESIS SUBMITTED
FOR THE DEGREE OF MASTER OF ENGINEERING
DEPARTMENT OF CHEMICAL AND BIOMOLECULAR
ENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE
2006

i
ACKNOWLEDGEMENTS

I would like to take this opportunity to express my deepest gratitude and thanks to my
supervisor Dr Rajasekhar Balasubramanian for his patience, guidance and support
throughout the course of this project.
I would like to thank Dr. Karthik for his advices and the directions given to me constantly
throughout the whole project, in addition my heartfelt gratitude goes to my groupmates
and other university staff in ESE and Chemical department.
I also wish to thank all my collegues working in INVISTA and several former collegues
from Regional Research Laboratory (RRL),Council of Scientific and Industrial Research
(CSIR), Trivandrum for their guidance and help over the course of my studies
Last but not least, I would like to thank my wife, daughter and other family members and
friends for their understanding and help during the entire period of my part-time studies.

ii
ABSTRACT
The contamination of water by toxic heavy metals including chromium is a worldwide
problem. The release of chromium into the environment has become a seroius health
problem due to its toxicity. Increasingly strict discharge limits on chromium have
accelerated the search for highly efficient yet economically attractive or alternative
treatment methods for its removal. The use of low-cost and waste biomaterials as
adsorbents of dissolved metal ions has shown potential to provide economic solutions to
this global environmental problem.
Numerous studies on metal biosorption by brown seaweeds such as Sargassum have been
reported. However the applicability of green seaweeds such as Ulva has not been
extensively investigated yet for the removal of Cr(VI)/Cr(III), despite of its large
abundance in the natural environment. In this study, laboratory scale investigations were
conducted to compare the adsorption capabilities of Ulva with Sargassum for the removal
of both Cr (VI) and Cr(III) from aqueous solutions. Various chemical pre-treatment
methods were investigated for enhancing the adsorption capacity of both Sargassum and
Ulva together with the use of other low cost waste biomaterials such as used tea and
coffee dust.
The most influencing adsorption parameters such as initial pH, quantity of adsorbent,
initial metal ion concentration and contact time were studied for Sargassum, Ulva, used
tea and coffee dusts. The adsorption capacity of Ulva was lower compared to that of
Sargassum. The removal of hexavalent chromium by seaweeds was observed as a process
of adsorption together with reduction by different kinetic rates. Ulva biomass only

iii

reduced less than 20% of the available Cr(VI) ions compared to a 100% reduction by
Sargassum. However, Ulva and Sargassum have shown similar adsorption capacities for
the removal of Cr(III) ions.
Experiments were conducted by using an external reducing agent to speed up the
reduction process by which an enhancement in the adsorption of Cr(VI) by Ulva biomass
could be achieved. Domestic wastes such as used tea and coffee dusts have been found to
be a strong anit-oxidant and be able to reduce more than 90% Cr(VI) ions to Cr(III) ions
within an hour. Adsorption experiments showed that used tea and coffee dusts are not only
good anti-oxidants, but also potential adsorbents which have a better adsorption capacity
than Sargassum.

iv
TABLE OF CONTENTS
Page No.


ACKNOWLEDGEMENT i

ABSTRACT ii

TABLE OF CONTENTS iv

LIST OF FIGURES vii

LIST OF TABLES x

NOMENCLATURE xi


CHAPTER 1 INTRODUCTION 1

CHAPTER 2 LITERATURE REVIEW 6

2.1 Conventional Chromium Removal Processes. 6
2.2 Biosorption 11
2.3 Use of Seaweed as Biosorbent 13
2.4 Biosorption Enhancement by Chemical Pre-treatment 15
2.5 Biosorption for the removal of Chromium 16
2.6 Reduction of Cr(VI) ot Cr(III) 16
2.7 Use of low cost biomaterials as antioxidant and adsorbent 18

CHAPTER 3 MATERIALS AND METHODS 20

3.1 Reagents used 20
3.1.1 Standard solutions of Chromium.
3.1.2 Hydrochloric acid (0.1N)
3.1.3 Sodium Hydroxide (0.1N)
3.1.4 Sulfuric acid,10% (v/v)

v
3.1.5 Diphenyl carbazide solution
3.1.6 Ferrous ammonium sulphate (500 ppm solution)
3.1.7 Hydroxylamine hydrochloride (10% solution)
3.1.8 Ascorbic acid (1% solution)
3.1.9 1 N Sodium hydroxide solution
3.1.10 Formaldehyde (1:2 vol% of Formaldehyde solution)
3.1.11 Acetone (50% (v/v) Acetone solution)
3.2 Biomaterials 21
3.2.1 Sargassum
3.2.2 Ulva

3.2.3 Modified Biomass
3.2.4 Tea and coffee dust
3.3 Biosorption studies 23
3.3.1 Effect of solution pH
3.3.2 Effect of initial concentration
3.3.3 Kinetics of chromium adsorption
3.4 Analytical methods 26
3.4.1 Total chromium concentration
3.4.2 Analysis of Chromium(VI) ions
3.4.2.1 Calibration of the unit
3.4.2.2 Sample preparation

3.5 Adsorption Isotherms 28
3.5.1 Langmuir Isotherm

vi
3.5.2 Freundlich Isotherm

CHAPTER 4 RESULTS AND DISCUSSION 31

4.1 Biosorption of Chromium(VI) and Chromium (III) by
Sargassum and Ulva. 31
4.1.1 Effect of pH
4.1.2 Effect of varying Biomass concentration
4.1.3 Effect of initial metal ion concentration.
4.1.4 Kinetics of Cr(VI) and Cr(III) adsorption.
4.1.5 Adsorption Isotherm for Sargassum and Ulva
4.2 Adsorption of Cr(VI) by pre-treated Biomass 43
4.2.1 Isotherm Analysis.
4.3 Reduction of Cr(VI) to less toxic Cr(III) 53

4.4 Use of coffee/tea waste for Cr(VI) reduction and removal. 60
4.4.1 Effect of pH
4.4.2 Bio-sorbent quantity optimization.
4.4.3 Effect of initial metal ion concentration.
4.4.4 Metal removal as a function of time.
4.4.5 Adsorption Isotherms.

CHAPTER 5 CONCLUSION 74

RECOMMENDED FUTURE STUDIES 77

REFERENCES 78

vii
LIST OF FIGURES
Page No.
Figure 4.1 Effect of pH on adsorption of Cr(VI)ions by Sargassum and Ulva. 31
Figure 4.2 Effect of pH on adsorption of Cr(VI)ions by Sargassum and Ulva. 33
Figure 4.3 Effect of varying initial concentration of Cr(VI) and Cr(III) ions on
biosorption by Sargassum and Ulva. 36
Figure 4.4 Kinetics of Cr(VI) adsorption by Sargassum and Ulva. 37
Figure 4.5 Kinetics of Cr(III) adsorption by Sargassum and Ulva. 39
Figure 4.6 Kinetics of Cr(VI) adsorption and reduction by Sargassum of Cr(VI) 40
Figure 4.7 Experimental sorption isotherm for adsorption of Cr(VI) and Cr(III)
ions from aqueous solution by seaweeds. 41

Figure 4.8 Percentage adsorption of Cr(VI) and Cr(III)ions by chemically
modified biomass of Sargassum and Ulva. 43

Figure 4.9 Adsorption uptake of Cr(VI) and Cr(III) ions by chemically modified

biomass of Sargassum and Ulva. 45
Figure 4.10 Experimental isotherm for Cr(VI)sorption by the unmodified and
pre-treated biomass of Sargassum. 48

Figure 4.11 Experimental isotherm for Cr(VI)sorption by the unmodified and
pre-treated biomass of Ulva. 49

Figure 4.12 Experimental isotherm for Cr(III) sorption by the unmodified and
pre-treated biomass of Sargassum. 49

Figure 4.13 Experimental isotherm for Cr(III) sorption by the unmodified and
pre-treated biomass of Ulva. 50

Figure 4.14 Adsorption and Reduction of Cr(VI) ions by Sargassum. 53
Figure 4.15 Total Cr(III) and Cr(VI) concentration Vs Time
Adsorption/Reduction by Sargassum. 54

Figure 4.16 Kinetic study for the adsorption of Cr(III)and Cr(VI) ions using
Sargassum. 55


Figure 4.17 Kinetic study for the adsorption of Cr(VI)ions using Sargassum
and Ulva 55

viii
Figure 4.18 Kinetic study for the adsorption of Cr(VI) ions by Sargassum
and Ulva. 56
Figure 4.19 Kinetic study for the adsorption of Cr(VI) ions by Sargassum and
Ulva. 57

Figure 4.20 Kinetics of the reduction of Cr(VI) ions by Tea and Coffee dust. 60
Figure 4.21 pH optimization for Adsorption/Reduction of Cr(VI) by tea dust. 60
Figure 4.22 pH optimization for Adsorption /Reduction of Cr(VI) by Coffee dust. 61
Figure 4.23 Effect of Biomass quantity for the Reduction/Adsorption of Cr(VI)
ions by tea dust. 63

Figure 4.24 Effect of Biomass quantity for the Reduction/Adsorption of Cr(VI)
ions by coffee dust. 63

Figure 4.25 Effect of Adsorption dose on uptake of Cr(VI) ions by tea and coffee
dust. 64

Figure 4.26 Effect of varying initial concentration of Cr(VI) for the Reduction
/Adsorption by Tea dust. 65

Figure 4.27 Effect of varying initial concentration of Cr(VI) for the
Reduction/Adsorption of coffee dust. 66

Figure 4.28 Comparison of Cr(VI) uptake capacities of Tea and Coffee dust at
different initial concentration of metal ion solution. 66

Figure 4.29 Kinetic study for the Reduction /Adsorption of Cr(VI)ions by Tea
dust. 67

Figure 4.30 Kinetic study for the Reduction /Adsorption of Cr(VI)ions by Coffee
dust. 68

Figure 4.31 Adsorption Isotherm for Cr(VI) adsorption by Tea and coffee dust. 69
Figure 4.32 Adsorption Isotherm for Cr(VI) Adsorption by Tea and coffee dust. 70
Figure 4.33 Percentage Reduction /Adsorption Cr(VI) by different biomaterials. 71

Figure 4.34 Percentage Adsorption for Cr(VI)by different biomaterials. 72
Figure 4.35 Percentage Reduction of Cr(VI) ions by Sargassum, used tea and dust 73


ix
LIST OF TABLES



Table 4.1. Adsorption parameters of pre-treated biomass for Cr(VI)
and Cr(III). 51

Table 4.2. Freundlich and Langmuir model isotherm constants for
Cr(VI)adsorption for Tea and Coffee dust.
70

x
NOMENCLATURE

C
0
- Initial concentration
C
e
- Equilibrium concentration (mg L
-1
)
C
eq
- Final equilibrium concentration (mg L
-1
)
Cr - Chromium
C

t
Concentraion at any time
GAC - Granular activated carbon
k - Langmuir equilibrium constant, related to the affinity of the
binding sites (L mg
-1
)

[H]
add
- Concentration of added acid
K
F
- Measure of adsorption capacity (Frendluich)
MCL - Maximum Contaminate Level
mg L
-1
- Milligram per litre
1/n - Adsorption intensity


[OH]
add -
Concentrations of added base
q
eq
- Mass of adsorbate adsorbed per unit mass of adsorbent at final
equilibrium concentration (mg g
-1
). This is also described as the

surface coverage.
q
max
- Maximum adsorption capacity (mg g
-1
)
q - Amount of metal ions adsorbed at equilibrium (mg g
-1
)
psi - Pounds per Square Inch
V - Volume of sample
W - Weight of adsorbent in gram
Sargassum - Sargassum Sp.
Ulva - Ulva Fasciata Sp.


1
CHAPTER 1
INTRODUCTION
Heavy metals can be defined as metallic elements with an atomic weight greater than that of
iron (55.8 g mol
-1
), or as elements with a density greater than 5 g cm
3
(Schuurman and Marker,
1997). Concern about heavy metals is due primarily to their potential toxicity, persistence, and
tendency to become concentrated in food chains (bioaccumulation).
Human exploitation of world’s mineral resources and advances in industrialization has resulted
in the presence of high levels of heavy metals in the environment. The presence of heavy
metals in the environment causes adverse impacts on flora and fauna of the earth. Though

many metallic elements are essential for nutritional and physiological requirements in living
organisms, their overabundance can cause toxicity symptoms, or even death. There are a
number of toxic heavy metals including chromium, whose increasing levels in the environment
are of considerable concern. With the rapid development of various industries, wastes
containing metals are discharged directly or indirectly into the environment. This trend has
been increasing, especially in developing countries, and has brought serious environmental
pollution and threatening to bio-life (Wang and Chan, 2006). Heavy metal pollution is arising
from effluent discharges from a variety of industries such as mining, ore processing, metal
processing operations, and industrial activities that make use of metallic compounds such as
pigments, bio-acidic agents, tanning, electroplating textile dyeing etc.
The toxic characteristics of heavy metals can be summarized as follows: (1) the toxicity can
last for a long time in nature; (2) some heavy metals such as chromium, arsenic, mercury etc.


2
could be transformed from relevant low toxic species into more toxic forms in a certain
environment; (3) the bioaccumulation and bio-augmentation of heavy metals by food chain
could affect normal physiological activity and endanger human life finally; (4) metals can only
be transformed and changed in valence and species, but cannot be degraded by any methods
including bio-treatment; (5) the toxicity of heavy metals occurs even in low concentration of
about 1.0–10 mg L
-1
. Some strong toxic metal ions, such as Hg, Cd and Cr, are very toxic even
in lower concentration of 0.001–0.1 mg L
-1
(Volesky, 1990a; Alkorta et al., 2004; Park et al.,
2005). Due to their increasing application and the above immutable nature, the heavy metal
pollution has naturally become one of the most serious environmental problems today.
Chromium is a metal found in natural deposits as ores, and also found in several other natural
materials in its compound form. The greatest use of chromium is in metal alloys such as

stainless steel; protective coatings on metal; magnetic tapes; and pigments for paints, cement,
paper, rubber, composition floor covering and other materials and its soluble forms are used in
wood preservatives (USEPA). Chromium may exist in several chemical forms and valence
states in the environment. The most commonly occurring valence states are chromium metal
(Cr(0)), trivalent Chromium (Cr(III)), and hexavalent Chromium (Cr(VI)). Chromium has been
used in electroplating, leather tanning, metal finishing, and chromate preparation industries
(Barnhart, 1997). Among its several oxidation states (e.g., di-, tri-, penta-, and hexa-), trivalent
(Cr
3+
and CrOH
2+
) and hexavalent (HCrO
4
-
and Cr
2
O
7
2-
) species of chromium are mainly found
in industrial effluents (Park et al., 2005). It is interesting that these two species of chromium
exhibit very different toxicities and mobilities in the environment. Cr(III) is relatively insoluble
at pH over 5 in aqueous systems and exhibits little or no toxicity (Anderson, 1997). In contrast,
Cr(VI) is highly soluble and toxic, which is suspected to be a carcinogen and mutagen (Costa,


3
2003). The human health effects caused by Cr(VI) are lung cancer, respiratory irritation,
dermatosis, dermatitis, and kidney and liver damage.
In the US alone, the production of the most water soluble forms of chromium, the chromate

and dichromates, was in the range of 250,000 tons in a year. Though chromium occurs in
nature mostly as chrome iron ore and is widely found in soils and plants, it is rarely found in
natural waters. The two largest sources of chromium emission in the atmosphere are from the
chemical manufacturing industry and combustion of natural gas, oil, and coal. When released
to land, chromium compounds bound to soil are not likely to migrate to ground water. They are
very persistent in water as sediments. Its concentrations in industrial waste waters range from
0.5 to 270,000 mg L
-1
(Patterson 1985). There is a high potential for accumulation of chromium
in aquatic life.
Chromium is also unique among regulated toxic elements in the environment in that different
species of chromium, specifically Cr(III) and Cr(VI), are regulated in different ways based on
their differing toxicities. Due to the severe toxicity of Cr(VI), the US EPA has set the
Maximum Contaminate Level (MCL) for Cr(VI) in domestic water supplies to be 0.05 mg L
-1
,
while total Cr containing Cr(III), Cr(VI) and other species of chromium is regulated to be
discharged below 2 mg L
-1
(Baral and Engelken, 2002). In Singapore the Environmental
Pollution Control Act restricts the release of Chromium to different water courses. The current
limit of Cr in all forms (trivalent and hexavalent) is 1 mg L
-1
for watercourse and 0.05 mg L
-1

for controlled watercourse (NEA, Singapore).
Due to the increasing awareness of the deleterious ecological and health effects of toxic metals,
a number of treatment methods have been developed over the years for their removal from

4
aqueous solutions. These methods mainly include reduction, ion exchange, electro-dialysis,
electrochemical precipitation, evaporation, solvent extraction, reverse osmosis, chemical
precipitation and adsorption (Patterson, 1985). However, these processes appear to be
ineffective or extremely expensive, especially when the dissolved metals are at low
concentrations (1 to 100 mg L
-1
) (Volesky, 1990). Some of the disadvantages associated with
the use of these methods include incomplete metal removal, high capital investment and
operation costs, and loss of efficiency during regeneration process. Natural biomaterials such
as seaweeds are available in large quantity. Certain waste products from industries, domestic,
or agricultural operations also, have great potential to be used as inexpensive sorbents. The
goal of the current study is to identify a suitable low cost biosorbent for the effective removal
of Cr from aqueous solutions.
Motivation
There are several developments in the biosorption studies for the removal of Cr ions from
various water sources. Sargassum was studied extensively for the removal of heavy metals
including Cr. However the applicability of green seaweed biomass such as Ulva for metal
removal has not been extensively investigated yet despite its large abundunce in the world’s
seashores. Chemical pre-treatments were studied for enhancing the adsorption capacities of
biosorbents, but limited studies were conducted for the modification of Ulva biomass. Very
few researchers have studied the effect of pre-treatment in Ulva in order to make this material
as a comparable biosorbent with Sargassum. Several studies on the biosorption mechanisms of
Cr(VI) adsorption proved that the mechanism is through the reduction and adsorption of Cr(III)
ions onto the biosorbents, but few studies were conducted to make use of an external reducing


5
agent for instantaneous reduction of Cr(VI) to Cr(III) to enhance the adsorption mechanism

using seaweeds (Katrochvil et al., 1998). Several studies have reported the effectiveness of
biomaterials and bio-material based activated carbons for the adsorption of Cr ions by only
analyzing Cr(VI) concentration in aqueous solutions resulting in incorrect elucidation of Cr
biosorption. Cr(VI) was removed from aqueous solution systems by ‘anionic adsorption’.
However, it has been proved that Cr(VI) is easily reduced to Cr(III) by contact with organic
materials under acidic conditions because of its high redox potential value (above +1.3V at
standard condition). Thus, it is quite possible that the mechanism of Cr(VI) removal by
biomaterials, or biomaterial-based activated carbons is not “anionic adsorption” but
“adsorption-coupled reduction”. It is therefore very important to analyze Cr(VI) and total Cr
concentrations in aqueous solution during Cr adsorption studies. Several reports in the
literature pointed out that plant biomass has the capability of reducing and retaining chromium
species. However, no detailed investigations have been conducted to comparatively evaluate
two different biomasses for the adsorption and reduction of Cr species from aqueous solutions.
Objectives
The main objective of this current research is to compare the efficiency of low cost
biomaterials for the removal of Cr from aqueous solution. For that purpose, most commonly
used brown algae (Sargassum sp ), less studied green algae (Ulva fasciata sp.), and waste
coffee and tea dusts were evaluated. The additonal objective is to explore the possibility of
improving the overall adsorption capacities of these different biomaterials by chemical pre-
treatment and/or the use of chemicals or other biomaterials through enhancing the reduction of
Cr(VI) ions during the adsorption process.


6
CHAPTER 2
LITERATURE REVIEW
This section provides background information related to the development of heavy metal
removal processes and a review of the past studies done on the removal of Cr from aqueous
solutions. In addition, the theories and factors influencing biosorption are presented. The
purpose of this review is to discuss the current status of different treatment methods developed

for the removal of Cr.
2.1 Conventional Chromium Removal Processes
Physicochemical methods, such as chemical precipitation, chemical oxidation or reduction,
electrochemical treatment, evaporative recovery, filtration, ion exchange, and membrane
technologies have been widely used to remove heavy metal ions from industrial wastewater.
Precipitation is used as the treatment scheme to extract heavy metals from solutions by almost
75% of the plating companies (Cushnie, 1985). Precipitation of metals from contaminated
water involves the conversion of soluble heavy metal salts to insoluble salts that will
precipitate. Physical methods such as clarification (settling) and/or filtration will then remove
the precipitate from the treated water. This process requires adjustment of pH, addition of a
chemical precipitant, and flocculation. The most common precipitation methods in industries
are hydroxide (or lime) precipitation, sulphide precipitation and sodium borohydride
precipitation. In precipitation processes, a dissolved substance is forced to form a fine
suspension of solid particles in order to effect a solid-liquid separation. It is dependent upon
the theoretical solubility of the most soluble species formed and the separation of the solids


7
from the aqueous solution. Heavy metals are usually precipitated as the hydroxide by the
addition of lime (calcium hydroxide) or potassium hydroxide or sodium hydroxide. These
methods are relatively inexpensive and are useful for removing the bulk of the heavy-metal
ions. However, they are not suitable where final clarification is required.
Ion exchange technologies have been successfully applied by metal finishing industries for
several decades. The system that is most commonly used involves cation exchange resins to
remove metal ions from a waste stream. Ion-exchange resins are insoluble polymers that have
active ionogenic groups that are either permanently ionized, or capable of ionization or
acceptance of protons to form the charged site. The resin interacts with mobile ions of
opposite charge from the external solution. Ion exchange resins are capable of exchanging an
H
+

ion for a cation in the waste stream, or in the case of anion resins, an OH
-
ion for an anion
in the waste stream. The resin is regenerated by an acid (cation resin), or a base (anion resin),
when the exchangeable ions have been depleted. Ion exchange is a process in which ions are
exchanged between a solution and an insoluble solid which is usually a resin. Several types
of ion-exchange resins are commercially available and some of them exhibit a high
specificity for certain heavy metals, however, a high capital expenditure is usually required
in order to purchase and operate such a system. Current membrane processes have hindrance
because of limited flow-rates, instability of the membranes in salt and acid conditions and
fouling by inorganic and organic species (Volesky 1990 a ; Aderhold et al., 1996).
Reverse osmosis is an ex-situ separation process most commonly used in the desalination of
the water. However, in the past decades a particular effort has been made for the application of
reverse osmosis in the metal-finishing industry, with recovery of concentrated solutions of


8
metal salts and reuse of the water in cleaning. Reverse osmosis is aimed at separating water
from ionic solutes (metal salts for example) and macromolecules. Reverse osmosis is a
pressure driven reversal of the natural process of osmosis. In the osmosis process, water is
transferred through a semipermeable membrane from the waterside of the membrane to the
dilute solution side until an osmotic equilibrium is reached. In the reverse osmosis process, a
hydraulic pressure (typically from 200 to 1200 psi) is applied to the salt solution side. This
arrests, or reverses the flow of water through the membrane depending on whether the
pressure equals or exceeds the osmotic pressure. Three types of semipermeable membrane
materials can be used in the reverse osmosis units: cellulose acetate, hollow fibber polyamides,
and polyether / amide on polysulfone membranes (thin film composite). Up to a few years ago,
reverse osmosis membranes were made almost exclusively of cellulose acetate. But new thin-
film composite has gained more attention recently. The performance of reverse osmosis
depends on membrane composition and configuration, pressure, temperature and

concentration of the feed water, the ionic charge and size of the specific treated ions.
Electrodialysis is a mass separation in which electrically charged membranes and an electrical
potential difference are used to separate ionic species from an aqueous solution and other
uncharged components. More specifically, ionic materials are selectively transported within a
stack of closely spaced ion exchange membranes. The driving force is provided by voltage
from a rectifier and is imposed on electrodes at the two ends of the stack.
Evaporation is the use of an energy source to vaporize a liquid form from a solution, slurry, or
sludge. In electroplating, nonvolatile metal salts are concentrated in the evaporating water and
can be reused. Atmospheric evaporators operate at atmospheric pressure and release the


9
moisture to the environment. Vacuum evaporators are also used and vaporize water at lower
temperatures. The Cart marker process (EPA, 1987) is an example of an atmospheric
evaporator. Chromic acid additions were reduced by about 95% and the waste treatment by
sodium bisulphate was eliminated. On the other hand, the cadmium platter process is an
example of the vacuum evaporator and is used to recover cadmium salts from a cadmium
cyanide plating system (EPA, 1987). Operating costs includes electrical power for the blower
and pump equipment, and heat for evaporation (usually 626 watts per litre or 3.371 watts per
gallon). Evaporation is an easy, maintenance-free, reliable and commonly applicable process.
The main disadvantages are high-energy consumption and undesirable constituents in the
recycled bath.
Carbon adsorption is a separation technology used to remove and recover certain inorganic
compounds from single-phase fluid streams. Granular activated carbon (GAC) is used as the
adsorbent. Activated carbons consist of amorphous forms of carbon that have been treated to
increase the surface area/volume ratio of the carbon. The most widely used activated carbons
are F-400 activated carbon from Calgon, which is made from bituminous material, and rice-
hull activated carbon (RHAC). Some batch experiments have compared the heavy metal
removal efficiency of the two types of GAC (Kim and Choi, 1998). The activated carbon F-
400 was reported to effectively remove chromium and lead, but did not remove cadmium,

while the rice-hull successfully removed cadmium and lead, but did not remove chromium.
Adsorption processes are versatile in terms of apparatus and offer a relatively simple method
for the removal of components, or impurities from liquid, or gaseous media. The absorbent
has to have the capability to selectively condense or concentrate the targeted adsorbate
(molecules, atoms, ions or particles) on its surface. Industrially important adsorbents include


10
activated carbon, silica gel, and alumina, which all have a porous surface structure and thus a
high surface area. Accordingly, there is no requirement to use or maintain living organisms for
the process. The process advantages of selecting non-viable biomasses have led to
considerable research into the use of these systems for the removal of heavy-metal ions
(Avery and Tobin, 1992; Orhan and Buyukgungor, 2000). The fact that such a broad range of
biomasses have been shown to exhibit some affinity for heavy metals indicates that the use of
cheap (or even waste) biomasses could be a future adsorbent in pollution control.
Chemical precipitation and electrochemical treatment are ineffective, especially when metal
ion concentration in aqueous solution is as low as 1 to 100 mg L
-1
(Volesky, 1990 a ;
Volesky, 1990 b). They also produce large amount of sludge to be treated with great
difficulties. Ion exchange, membrane technologies, and activated carbon adsorption process
are extremely expensive, especially when treating a large amount of water and wastewater
containing heavy metals in low concentration, so they cannot be used at large scale. In
addition, they often create secondary problems since they give rise to metal bearing sludges.
Alternative technologies termed biosorption have been extensively studied in the last two
decades, and are based on the metal-sequestering properties of certain natural biomasses, such
as fungi, bacteria, and algae. These biosorbents possess metal-sequestering properties and can
decrease the concentration of heavy metal ions in solution from ppm (parts per million) to ppt
(parts per trillion) level. They can effectively sequester dissolved metal ions out of dilute
complex solutions with high efficiency and quickly. Biological methods such as biosorption/

bioaccumulation for the removal of heavy metal ions may provide an attractive alternative to
physicochemical methods (Kapoor and Viraraghavan, 1995).


11
Wide ranges of low cost adsorbents were studied for the removal of heavy metals. These
include sorbents such as bark/tannin-rich materials, lignin, chitin/chitosan, dead biomass,
seaweed/algae/alginate, xanthate, zeolite, clay, fly ash, iron-oxide-coated sand etc (Williams et
al., 1996; Susan et al., 1999). Sorption depends heavily on experimental conditions such as pH,
metal concentration, ligand concentration, competing ions, and particle size (Susan et al., 1999),
so it is important to study these parameters during the biosorption experiments.
2.2
Biosorption
The biological method for metal remediation involves the use of living or non-living
adsorbents. The use of living organisms is known as bioaccumulation, and this process has
some practical difficulties. The industrial effluents usually contain high concentrations of toxic
metals, and the pH conditions are usually extremely high or low. These conditions are not
congenial for the survival of living cells. The living organisms have to be maintained in good
physiological condition by providing a constant energy source. Finally, the acid and alkaline
eluents used for metal recovery may kill the microorganisms and living cells. Therefore, this
process is not feasible for all de-toxification procedures. On the other hand, biosorption by
using dead biomass becomes an emerging technology, and it was recognized as a potential
alternative for recovery of heavy metals from industrial waste streams.
Biosorption is based on the following mechanisms: physical adsorption, ion exchange,
complexation, and precipitation. Biosorption may not necessarily consist of a single
mechanism. In many sorption processes, more than one of these mechanisms take place, and it
is difficult to distinguish between the single steps (Lacher and Smith, 2002). The major
advantages of biosorption include: (i) low cost, (ii) high efficiency of heavy metal removal

12
from diluted solutions, (iii) regeneration of the biosorbent, and (iv) the possibility of metal
recovery. In the last several years considerable attention has been focused on biosorption of
metal ions from aqueous effluents (Holan and Volesky, 1995).
Recent investigations by various groups have shown that selected species of seaweeds possess
impressive sorption capacities for a wide range of heavy metal ions. Seaweeds are a widely
available source of biomass as over two million tones are either harvested from the oceans, or
cultured annually for food or phycocolloid production, especially in the Asia-Pacific region
(Zemke and Ohno, 1999). Among the different biological substrates tested in biosorption
studies, algal biomass has received considerable attention due to the cost saving, low
sensitivity to environmental and impurity factors, the possible contaminant recovery from the
biomaterial, and its elevated adsorption capacity. This applies to Sargassum and Ulva, which
are available in large quantities in littoral zones and therefore, an inexpensive sorbent material.
Among the most promising biomaterials studied, seaweeds are found to be very efficient and
bind a variety of metals (Holan and Volesky, 1995). Many types of biomass have been reported
to have high uptake capacities for heavy metals, including Cr. Among these materials, some
species of brown marine algae exhibit much higher uptake values than other types of biomass,
higher than activated carbon and comparable to those of synthetic ion exchange resins. The
presence of key functional groups on the algal cell walls is responsible for their outstanding
metal-sorbing properties (Davis et al., 2003). Marine algae, popularly known as seaweeds, are
biological resources and are available in large quantities in many parts of the world. The algal
cell wall of marine algae contains a high proportion of alginate constituting 14–40% of the dry


13
weight of the biomass. Alginic acid is a polymer composed of un-branched chains of 1, 4-
linked h-d-mannuronic and a-l-guluronic acids (Percival and McDowell, 1967).
2.3 Use of Seaweed as Biosorbent
Among the various seaweeds investigated, Sargassum Sp. possesses superior metal binding
capacity (Holan and Volesky, 1995). Sargassum is a type of brown algae, and it is abundantly

found along the coast of beaches. The cell wall constituents and the porosity of the cell wall
play an important role in biosorbent metal uptake and binding (Volesky, 1990a). Different
types of algae have different cell constituents, composition and structure. Sargassum contains
high amount of alginate within its cellular structures. These alginates are common to brown
seaweeds, and the carboxyl groups of uronic acids (guluronic, mannuronic, glucuronic) in the
alginate are the dominating binding groups (Volesky, 2003a). The alginate matrix is in a gel
phase and hence makes it easily penetrable for small metallic cations; this makes it a suitable
biosorbent with a high sorption capacity (Siegel and Siegel, 1973).
Sargassum species are found throughout tropical areas of the world and are often the most
obvious macrophyte in near-shore areas where Sargassum beds often occur near coral reefs.
Sargassum constitutes about 10 ± 40% of the brown algal dry weight (Percival and McDowell
1967), between 17% and 45% alginate contents (Chapman 1980; Fourest and Volesky 1996)
which corresponds to 0.85 ± 0.25 mequiv/g of carboxyl groups per dry weight. Brown algae
also contain about 5 ± 20% of the sulfated matrix polysaccharide fucoidan (Chapman, 1980)
about 40% of which are sulfate esters. Fourest and Volesky (1996) reported that 0.27 mequiv/g
of sulfate groups are found in Sargassum seaweed.